Presently, wireless communication devices are being configured with multiple transmitters and receivers. A wireless communication device can transmit multiple signals simultaneously, but doing so causes multiple order intermodulation distortion to the signals received by the device. With multiple receivers operating simultaneously, there can be desensitization if a receive channel frequency is on a harmonic frequency of a single transmitter or on an intermodulation frequency of multiple transmitters. In order to reduce the desensitization caused by transmitters, the devices often impose the front end components of the device to have high linearity. As devices increase on complexity and the number of transmitters and receivers increase, it is challenging to configure existing front end components such as radio frequency switches, power amplifiers and duplex filters, with sufficient linearity.
Certain wireless communication devices are required to perform simultaneous transmissions for different radio access networks and modes. In these products, the intermodulation distortion signal generated from the at least two transmit signals can fall within the receive band of the received signal causing receiver desensitizing interference. In certain cases, this distortion can be odd-order, e.g. 3rd and 5th order, intermodulation in the receive bands. Also, in the case of a single transmitter and multiple receivers, it is possible that a transmitter harmonic distortion falls into a receive band. The problem of harmonic or intermodulation distortion causing receiver desensitization may continue to exist as the market adopts new features as accepted by 3GPP and other standards that use multiple carriers.
In the past, a solution to the harmonic or intermodulation distortion was to reduce transmitter power. For the case of 3rd order distortion, the interference is reduced 3 dB for every 1 dB of transmitter power. Unfortunately, the reduced power reduces uplink performance. Another solution is to isolate the circuits that are carrying the transmit signals in the device. One way to achieve this is to use a single antenna and combine the transmit signals with a diplexer having sufficient isolation. Alternatively, the signals can be isolated by using separate antennas with sufficient isolation between them. Another solution could also employ both separate antennas and diplexing circuits. Unfortunately, these approaches require large and costly additional Radio-Frequency (RF) components and do not provide sufficient isolation to fully eliminate receiver desensitization. RF cancellation methods have also been proposed. Unfortunately, these cancellation methods require additional front end components that increase the cost, size and power dissipation of the devices.
Another method is to cancel the intermodulation distortion in the receiver baseband. In one such endeavor, a baseband cancellation method separates the interfering signal from the receive signal in order to generate a replica intermodulation distortion signal. This is done by extracting the distortion signal from the actual received signal. Unfortunately, the extraction of interference from the received signal requires additional calibration hardware and software. One can also generate baseband intermodulation distortion signals directly from the baseband transmitter. In the past this method has been applied to cases where there is a single transmit signal with second order intermodulation distortion causing an intermodulation signal at baseband. Unfortunately, this method does not work for multiple transmit signals because it does not address the problem of intermodulation distortion occurring at non-direct current (DC) baseband frequency, such as at a correct low intermediate frequency.
In view of the foregoing, there is a need to address higher order harmonic and intermodulation distortion which occur on a received signal frequencies, caused by one or more transmitters.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.